Web compression buckling capacity prediction for unstiffened I-sections with opposite patch loading Fatmir Menkulasi a, ⁎, Nahid Farzana b a Department of Civil and Environmental Engineering, Wayne State University, Detroit, MI 48202, USA b Department of Civil Engineering, Louisiana Tech University, Ruston, LA, USA abstract article info Article history: Received 12 March 2019 Received in revised form 8 June 2019 Accepted 8 August 2019 Available online xxxx Two web compression buckling capacity prediction methods are introduced for unstiffened steel I-sections sub- ject to opposite patch loading applied to the flanges. The methods are generally posed as a function of loaded width to web depth ratio, and are applicable for opposite patch loading applied at the interior of a wide flange section or at the end of it, where the web has a free edge. The proposed methods include three parts: 1) an ex- pression for predicting the squash load, 2) an expression for predicting the elastic buckling load, and 3) a resis- tance function. The squash load is calculated using an empirically derived effective width concept based on observations at the ultimate load from an extensive experimental database and validated numerical simulations. Web slenderness is defined as the square root of the ratio of the web squash load to the web critical elastic buck- ling load. The critical elastic buckling load is defined consistently with that obtained with a plate buckling energy solution for patch loading on infinitely long strips and considers the shortened web buckling half-wavelength resulting from flange rotational restraint provided to the web. The methods are validated with existing experi- mental data and shell finite element collapse simulations, and are shown to be more accurate and more widely applicable than current American Institute for Steel Construction (AISC) Specification provisions. © 2019 Elsevier Ltd. All rights reserved. Keywords: Opposite patch loading Web compression buckling Critical elastic buckling Finite element analysis Capacity prediction 1. Introduction The research presented in this paper deals with the behavior of unstiffened steel I-sections subject to opposite patch loading away or at member ends (Fig. 1a and b). Fig. 1c provides some practical exam- ples of steel I-sections subject to opposite patch loading. One example is a through-girder in which the column above and the column below align but the girder needs to cantilever over the column below for var- ious detailing reasons. Another example is a beam-column moment connection under gravity loads, which features beams framing on both sides of the column. Research on opposite patch loading resistance of steel plates dates back to the early 1900s with some researchers dealing primarily with the elastic stability of steel plates and some others with their ultimate resistance. The problem of elastic stability of a simply supported rectan- gular plate subject to opposite patch loading was first attempted by Sommerfield [1] and later by Timoshenko [2] who used a strain energy approach to develop an approximate solution by neglecting the exten- sional deformation of the middle surface during buckling. Leggett [3] developed an accurate solution for the elastic stability of infinitely long plates subject to equal and opposite concentrated forces, and Hop- kins [4] specialized the general problem of opposite patch loading to that of sets of concentrated normal forces, and to that of a normal force distributed uniformly over a finite length. Subsequently, Yamaki [5] investigated the elastic stability of a rectangular plate under opposite patch loading for the case when all edges are simply supported, and when loaded edges are simply supported while the other edges are clamped. Khan and Walker [6] and Lagerqvist and Johansson [7] pro- posed a value for the elastic buckling coefficient, k, which resulted in good capacity predictions. Winter and Pian [8] developed empirical formulas for predicting the crushing strength of steel thin webs in cold-formed steel members sub- ject to opposite concentrated loads. While they admit that such a purely empirical approach is likely to produce results of somewhat limited va- lidity, they also state that the range of variations of dimensions and shapes of test specimens considered is wide enough to cover the practi- cally important range of elements. Section J10 of AISC Specifications [9] addresses conditions in which flanges and webs are subject to concentrated forces and provides two equations for calculating the web compression buckling capacity of members subject to equal and opposite concentrated forces. However, in these equations, the loaded width is not a variable since these equa- tions were originally developed for directly welded beam-column mo- ment connections in which the concentrated loads come from the Journal of Constructional Steel Research 162 (2019) 105728 ⁎ Corresponding author. E-mail addresses: fatmir.menkulasi@wayne.edu (F. Menkulasi), nfa004@latech.edu (N. Farzana). https://doi.org/10.1016/j.jcsr.2019.105728 0143-974X/© 2019 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Constructional Steel Research